There have conventionally widely known a method of forming a circuit pattern by sticking copper foil on a substrate consisting of an epoxy resin containing glass fibers, or the like, masking the copper foil and etching the unmasked portions of the copper foil. In this method, it is necessary to secure a minimum circuit pattern width and pattern gap size for enabling processing for patterning by masking, but there is a limit in making the circuit pattern width smaller. Further, when electronic parts (light-emitting elements) are highly densely disposed on a circuit pattern, there is used an apparatus that carries such electronic parts, moves them to a circuit pattern, and disposes them at right positions determined by alignment. The gaps between adjacent electronic parts (light-emitting elements) must be larger than the alignment accuracy of such an apparatus as mentioned above, and therefore there is a limit in making the gap between electronic parts smaller.
Patent document 1 discloses a light source carrying, for the purpose of obtaining a highly bright LED illumination light source, a plurality of LED elements alternately mounted on every other substrate among two of overlappingly disposed mounting substrates, so that the LED elements are disposed in the shape of matrix when they are seen from above. As for two of the mounting substrates, a transparent substrate is used as the upper mounting substrate, so that the lights emitted by the LED elements carried on the lower mounting substrate transmit through the upper mounting substrate, and are projected upward together with the lights emitted by the LED elements carried on the upper mounting substrate.
As disclosed in Patent document 2, in the field of printed electronics, there has also been developed a method for forming a circuit pattern having smaller circuit pattern widths and pattern gaps by drawing (printing) a circuit pattern on a substrate by photogravure printing, flexographic printing, or the like with an ink mainly consisting of metal nanoparticles, and then irradiating the whole substrate with light to sinter the metal nanoparticles. In the method disclosed in Patent document 2, after a circuit pattern is printed with the conductive ink, a liquid medium contained in the ink is removed to a predetermined content (0.01 to 3 mass %) to prevent collapse of the circuit pattern until the sintering with light, and formation of holes in the inside of the circuit pattern at the time of the sintering with light.